In this lesson, students deepen their understanding of subtraction by relating the concrete model of subtraction to the standard algorithm. They model decompositions on their place-value chart while simultaneously recording the changes in the written form. Students draw a magnifying glass around the minuend, then ask the question, "Do I have enough ones?" They refer to the place-value disks to answer and exchange a ten disk for 10 ones when necessary. They record the change in the vertical form. Students repeat these steps when subtracting the tens.